verification of the potential yield and chemical quality

ISWS/CIR-149/81

Circular 149STATE OF ILLINOIS

DEPARTMENT OF ENERGY AND NATURAL RESOURCES

Verification of the Potential Yield and Chemical Quality

of the Shallow Dolomite Aquifer

in DuPage County, Illinois

by ROBERT T. SASMAN, RICHARD J. SCHICHT, JAMES P. GIBB,MICHAEL O'HEARN, CURTIS R. BENSON, and R. SCOTT LUDWIGS

ILLINOIS STATE WATER SURVEY

CHAMPAIGN

1981

CONTENTS

PAGE

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Scope of study . . . . . . . . . . . . . . . . . . . . . . . . 1Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Geology and hydrology . . . . . . . . . . . . . . . . . . . . . . . . . 4

Shallow aquifer pumpage . . . . . . . . . . . . . . . . . . . . . . . . 5

Piezometric surface, 1979 . . . . . . . . . . . . . . . . . . . . . . . 6

Changes in water levels, 1966-1979. . . . . . . . . . . . . . . . 8

Water level-bedrock surface relationship . . . . . . . . . . . . . . . . 12

Water level-basal sand and gravel relationship . . . . . . . . . . . . . 16

Water budget. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Total dissolved solids (TDS) . . . . . . . . . . . . . . . . . . . 25Hardness (as CaC03) . . . . . . . . . . . . . . . . . . . . . . . . 26Sulfate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Sodium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Total organic carbon . . . . . . . . . . . . . . . . . . . . . . . 36Fluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Total iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Nitrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . 43

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 46

Verification of the Potential Yield and Chemical Quality

of the Shallow Dolomite Aquifer

in DuPage County, Illinois

by Robert T. Sasman, Richard J. Schicht, James P. Gibb,Michael O'Hearn, Curtis R. Benson, and R. Scott Ludwigs

INTRODUCTION

Scope of Study

Because of its responsibility for allocating the Lake Michigan water whichIllinois is permitted to divert from the Great Lakes Basin, the IllinoisDivision of Water Resources must consider alternative sources of wateravailable to the Chicago metropolitan region. In early 1979, the Divisioncontracted with the Illinois State Water Survey to study the shallow dolomiteaquifer in DuPage County, an important alternative source, to verify itspotential yield. The plan of the investigation was to analyze the effects ofcontinually increasing pumpage, and to identify areas of existing and potentialproblems and areas where additional pumpage can be developed.

In the summer of 1979, as part of this study, a detailed data collectionprogram in the shallow dolomite was conducted over a 700-square-mile area,shown in figure 1. The area centered on DuPage County but also included partsof west and northwest Cook County, east Kane County, northeast Kendall County,and north Will County. It was bounded on the west by the Fox River and on theeast and southeast by the Des Plaines River. Data obtained included pumpagefrom major water users and more than 1200 water level measurements. Inaddition, 282 water samples were collected for subsequent analysis.

purposes:The data collected have provided a base of analysis for the following

•

•

•

•

To compare the 1979 piezometric surface map with an earlier piezo-metric map to show the effect of increased withdrawals on waterlevels.

To delineate areas of over-utilization and areas showing littleeffect from existing pumpage.

To compare the piezometric surface with the dolomite bedrock surfacein order to identify areas where dewatering of the dolomite has takenplace.

To prepare a water budget taking into account pumpage, the potentialyield, and the dewatering of both the dolomite aquifer and the over-lying sand and gravel.

1

Figure 1. Location map of study area and selected communities

2

• To prepare chemical concentration maps for TDS, hardness, sulfate,chloride, sodium, total organic carbon, fluoride, total iron, andnitrate.

• To explore possible causes for increased chemical concentrations on aregional and time basis.

Zeizel et al. (1962) provided the first detailed report on the groundwaterresources of DuPage County. A study by Sasman (1974) included a 1966piezometric surface map as well as pumpage and other data concerning thedolomite aquifer in DuPage County. The map and data in Sasman's report werecompared with data obtained during the present study in order to analyze thechanges that occurred during the 13-year interval.

Prickett et al. (1964) presented an analysis of the shallow dolomiteaquifer in the LaGrange area. This study also helped provide some comparisonswith present conditions. The data obtained in this and other sections outsideof DuPage County provided important basic information for these areas andinsured that all regions affected by pumpage within DuPage County would beincluded in the investigation. Rivers on the east and west sides of the studyarea helped to assure reasonable hydrologic boundaries.

Acknowledgments

Special recognition needs to be given to the numerous individuals andorganizations who provided access to their wells during the field work part ofthis project. Water levels were measured in more than 850 private, domesticwells, and water samples were collected from more than 100 of these wells.Water levels and water samples were collected from shallow dolomite wells inevery municipality that has shallow wells in the area.

Most of the water level and samples from domestic wells were obtained byDon Arnold, Tim Larson, (Ralph Laukant), and Rebeccah Prastein, four area collegestudents who were hired for the project. With no prior experience and only abrief orientation period, they performed an outstanding job, and the projectcould not have been completed in the allotted time span without their readyparticipation.

Special appreciation is also given to representatives of the Cook Countyand DuPage County Forest Preserve Districts, DuPage County Public WorksDepartment, Argonne National Laboratory, and Fermi National Laboratory forassisting with water level measuring and sampling of numerous wells on theirproperties.

Jeanette Prastein reviewed most of the students' measurement calculationsand tabulated much of the pumpage data. The Survey's graphic arts staff, underthe guidance of John W. Brother, Jr., prepared all the final figures, GailTaylor edited the final manuscript, and Debbie Hayn prepared the camera copy.

3

GEOLOGY AND HYDROLOGY

For a detailed discussion of the geology and hydrology of the aquifers innortheastern Illinois, the reader is referred to Suter et al. (1959) and Zeizelet al. (1962). The following section is based largely upon those two reports.

Unconsolidated deposits, mainly glacial drift ranging in thickness from afoot or less to slightly more than 200 feet, overlie the bedrock in the area.Groundwater in the drift is obtained from sands and gravels that occur assurface deposits or more commonly as deposits underlying or interbedded withglacial till. Moderate to large supplies of groundwater are primarilyencountered in sand and gravel at the base of the drift, directly abovebedrock. Buried sand and gravel deposits, some associated with buried bedrockvalleys, are present in northwestern Cook, northern DuPage, and northeasternKane Counties. Areas with drift less than 50 feet thick, and poorpossibilities for water-bearing sand and gravel, occur primarily in thesouthern and eastern part of the study area.

Groundwater in sand and gravel aquifers commonly occurs under leakyartesian conditions. Recharge to sand and gravel aquifers is derived mostly byvertical leakage of water from surface deposits through glacial drift depositsoverlying the aquifers. Surface deposits are recharged locally fromprecipitation. Glacial drift aquifers in large areas of northeastern Illinoisare in hydraulic connection with underlying shallow bedrock aquifers.

The bedrock immediately beneath the glacial drift consists almost entirelyof dolomite rocks of Silurian age. Small areas of the bedrock surface innorthern DuPage, eastern Kane, and northeastern Kendall Counties consist ofrocks of the Maquoketa Formation.

Rocks of Silurian age are the Alexandrian Series overlain by the NiagaranSeries. The thickness of the Silurian dolomite in the western part of the arearanges from less than 50 to 100 feet. It thickens to the east and southeastand is more than 300 feet thick in parts of west-central Cook County. Wherevalleys occur in the bedrock, the Silurian rocks are thin or missing. In manyareas, especially where these rocks are more than 100 feet thick, Siluriandolomite wells yield several hundred gallons per minute.

Groundwater in the shallow dolomite aquifer occurs in joints, fissures,and solution cavities. The water-bearing openings are irregularly distributedboth vertically and horizontally, and the yields of shallow dolomite wells varygreatly from place to place. Available geohydrologic data suggest that on aregional basis, the shallow dolomite is permeated by numerous fractures andcrevices which extend for considerable distances and are interconnected. Theserocks receive recharge from overlying glacial deposits, or directly from pre-cipitation where they are not covered by the drift. The upper part of theaquifer is usually the most productive.

The Maquoketa Formation consisting of shale and dolomite rocks ofOrdovician age underlies the Silurian rocks. The thickness of the MaquoketaFormation ranges from less than 100 feet in the western part of the area tomore than 200 feet in much of northern and eastern DuPage County and in CookCounty. Yields from this formation are usually very limited, although in some

4

areas the dolomite contributes moderate quantities of water to wellspenetrating both Silurian and Maquoketa socks. The relatively impermeableshales of the Maquoketa Formation act as a partial barrier to the downwardmovement of groundwater from the Silurian dolomite aquifer into the deeperCambrian-Ordovician aquifer.

The Cambrian-Ordovician and Mt. Simon Aquifers lie beneath the MaquoketaFormation and have been described in detail in many State Water Survey andGeological Survey reports including the report by Suter et al. (1959).

SHALLOW AQUIFER PUMPAGE

Combined dolomite and sand and gravel pumpage for public and major non-public supplies in the study area increased from 32.1 million gallons per day(mgd) in 1966 to 61.7 mgd in 1978, an increase of about 92 percent. Ninety-three percent of the 1978 shallow pumpage was from the dolomite aquifers.Table 1 shows the combined pumpage for the study area for the period 1966-1978.

County

Cook

DuPage

Kane

Kendall

Will

Total

County

Cook

DuPage

Kane

Kendall

Will

Total

Table 1 Shallow Aquifer Public and Major Non-PublicPumpage (mgd) in the Study Area, 1966-1978

1966 1967

6.90 7.54

24.00 24.86

.93 .94

.01 .02

.28 .31

32.12 33.67

1968

8.38

27.74

1.03

1971

10.71

34.14

1.30

.01

.51

37.67

1969 1970

8.67 9.48

28.74 31.62

1.02 1.08

.03 .03

.54 .52

39.00 42.73

.05

1.09

47.29

1973

11.79

38.39

1.54

.06

2.13

53.91

1974

11.79

36.24

1.42

.07

2.39

51.91

1975 1976 1977

12.36 11.80 11.52

39.64 42.95 44.39

1.44 1.36 1.34

.09 .09 .05

3.66 3.55 3.71

57.19 59.75 61.01

1978

10.78

45.79

1.30

.07

3.75

61.69

1972

11.73

35.12

1.47

.06

1.57

49.95

5

During this same period, pumpage in DuPage County increased from 24 to45.8 mgd, an increase of about 91 percent. Ninety-five percent of the 1978shallow aquifer pumpage in DuPage County, or 43.5 mgd, was from the dolomite.Schicht et al. (1976) calculated the potential yield of these shallow aquifersin DuPage County to be approximately 44.4 mgd.

Bensenville is the only municipality within the study area that has alwaysobtained all of its water from deep wells. Nine public water supplies inDuPage County (Addison, Glendale Heights, Glen Ellyn, Hinsdale, Lisle,Warrenville, Wheaton, Winfield, and Woodridge), three in Cook County (BurrRidge, Indian Head Park, and LaGrange Highlands), and one in Will County(Bolingbrook) have always obtained all of their water from shallow wells.Downers Grove and Itasca have obtained all of their water from shallow wellsfor more than 40 years. Batavia and Geneva, on the edge of the area in Kanecounty, obtain all of their water from deep wells. All of the othermunicipalities in the area obtain water from both shallow and deep wells inwidely ranging proportions. Figure 1 shows the general location of most of themunicipalities in the area pumping significant volumes of water from theshallow aquifers.

PIEZOMETRIC SURFACE, 1979

In order to determine areas affected by groundwater withdrawals anddirections of groundwater movement in the Silurian dolomite aquifer, apiezometric surface map was prepared from nonpumping water levels measured inwells during the summer of 1979 (figure 2). The map represents the elevationto which water will rise in a well completed in the Silurian dolomite aquifer.In a few instances, wells are open to basal sand and gravel deposits above thedolomite bedrock. The basal sand and gravel is in direct contact with thedolomite, and the two formations act as one hydrologic unit. The direction ofgroundwater movement is at right angles to the elevation contour lines.Groundwater movement in the entire area is in all directions from highelevations toward streams, well fields, and quarry dewatering areas (seefigure 2).

Before significant groundwater development, the groundwater levels weregenerally above surface water levels in rivers and streams, and groundwaterdischarged into these surface waters throughout the region. A significantportion of streamflow was derived from groundwater.

Due to groundwater withdrawals, water levels in numerous areas havelowered considerably below stream level, resulting in reductions in streamflow.In these areas surface water percolates through the stream bed and into thedolomite aquifer. In 1979, shallow dolomite water levels were above those ofthe surface waterways in only a few areas (along some reaches of the FOX,DuPage, and Des Plaines Rivers and Salt Creek).

Heavy, concentrated groundwater withdrawals have produced cones ofdepression in many parts of the area. The most significant cones of depression

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are at and adjacent to rock quarries at McCook-Lyons, Elmhurst, and Hillside.Dewatering pumpage at these quarries has been continuous for more than 80years.

In addition to the effects of quarry dewatering at McCook-Lyons andElmhurst, heavy well pumpage in these areas has also contributed greatly tolower piezometric surface elevations. Other significant cones of depressioncan be attributed to pumpage from wells around the communities of Itasca, WoodDale, Addison, West Chicago, and Glendale Heights. Pumpage has caused majorpiezometric surface cones of depression in a large area of southeast DuPageCounty. Numerous smaller cones of depression and distorted contours occurthroughout the area as a result of heavy local or regional pumpage.

The average slope of the piezometric surface ranges from a low of about 10feet per mile in areas of small to moderate pumpage to more than 25 feet permile near and within major cones of depression.

An analysis of the piezometric surface for 1979 indicated that about 68percent of the study area was influenced by groundwater withdrawals from theSilurian dolomite aquifer. Figure 3 shows approximate boundaries of areasinfluenced by pumpage and the total amount of pumpage from major wells and wellfields within the identified areas. In DuPage County about 80 percent of thearea was influenced by groundwater withdrawals.

CHANGES IN WATER LEVELS, 1966-1979

Comparison of the 1966 (figure 4) and 1979 (figure 2) piezometric surfacemaps for DuPage County and part of western Cook County indicates the waterlevel change during the 13-year period. Areas of water level change are shownin figure 5. More than 98 square miles, or 30 percent of DuPage County,experienced a decline of more than 10 feet. About 8.5 square miles haddeclines of more than 30 feet. In a few areas, declines of more than 50 feetwere recorded.

The most dramatic area of water level decline in DuPage County is in theGlendale Heights area in the north central part of the county. More than 3square miles have had a decline of over 30 feet. Other large areas ofsignificant decline are in the Carol Stream-Bloomingdale-Roselle area, theItasca-Wood Dale-Addison area, the Wheaton-Glen Ellyn area, and theHinsdale-Clarendon Hills-Westmont-Willow Brook-Darien-Burr Ridge area.Numerous other pumping centers throughout the county have experiencednoticeable declines during the past 13 years.

In the 98 square miles of DuPage County where water levels have declined10 feet or more, the average rate of decline during the past 13 years has beenapproximately 1.5 feet per year. For all of DuPage County, water levels havedeclined an average of about 0.5 feet per year.

8

Figure 3. Areas influenced by groundwater withdrawals in 1979

9

Figure 4. Piezometric surface of the shallow dolomite aquiferin DuPage County, 1966

10

Figure 5. Change in piezometric surface in DuPage County and part of Cook County, 1966-1979

11

An area of approximately 13 square miles in western Cook County also had awater level decline of more than 10 feet. In much of this area (6.3 squaremiles), the decline was more than 30 feet.

Water level declines in Cook County are primarily due to water with-drawals at the quarries and to withdrawals for public supplies at LaGrange,LaGrange Highlands, and Indian Head Park. Numerous additional public,industrial, and commercial groundwater supply systems have contributed to thelowering of water levels in this area. During the past 13 years, water levelsin this part of Cook County have declined at a rate of approximately 2.3 feetper year.

Comparison of the two piezometric maps (figures 2 and 4) indicates severalareas where water levels have risen more than 10 feet since 1966. In thevicinity of West Chicago and Elmhurst-Oak Brook, rises in water levels reflectdecreased withdrawals from dolomite wells. In other areas, indicated rises inwater levels may be due to collection of more detailed data in 1979 or tovariations in precipitation.

Water level measurements in individual wells also provide an indication ofwater level fluctuations over a period of time. Water level fluctuations inshallow dolomite and sand and gravel wells are influenced by a number offactors, including frequency and duration of the pump operation cycle, pumpingrate, influence from other wells in the area, hydrologic and geologiccharacteristics of the rock formation, and short- and long-term variations inweather conditions (recharge).

Figures 6, 7, and 8 present water level hydrographs of several wells forperiods of 7 to 20 years. Water levels in 6 of these wells are measuredmonthly or recorded on continuous recording instruments. They showconsiderable variation, ranging from slight rises or essentially no change in afew wells to a decline of more than 100 feet in a well near LaGrange (Figure7). Five of the selected wells have average declines ranging from 1.2 to 8.5feet per year for the period 1966-1979. Four others have average declines of0.1 to 0.9 feet per year. Four wells have no significant change of water levelduring recent years.

WATER LEVEL-BEDROCK SURFACE RELATIONSHIP

Past experience indicates that a great potential exists for a significantdecline in well yields when the water level lowers below the top of thedolomite. Previous analysis of well production tests by Zeizel et al. (1962)indicates a higher yield per foot of formation in the upper part of thedolomite aquifer. Data suggest that at least 12 communities in themetropolitan area have experienced intermittent or continuous loss of capacity

12

Figure 6. Hydrographs of water levels in selected wells

13


14


15

as a result of partial aquifer dewatering. Nearly all of the communities arein the west suburban area, and most are in DuPage County. They are:

Addison LisleClarendon Hills Oak BrookDowners Grove SchaumburgGlen Ellyn Villa ParkHinsdale West ChicagoItasca Wood Dale

To determine areas where the dolomite has been dewatered, piezometricsurface maps were compared with the map of the bedrock surface (figure 9). The1979 water level-bedrock surface relationship map is shown in figure 10. Waterlevels have been lowered below the top of the rock over more than 77 squaremiles, or 11 percent of the entire area. Water levels are more than 50 feetbelow the bedrock surface for over 14 square miles and more than 100 feet belowthe bedrock surface for over 6 square miles. An average thickness of nearly 39feet of rock has been dewatered for 77 square miles.

The largest areas and thickest sections of dewatered dolomite areassociated with the quarry operations in the east and southeast parts of theregion. Heavy well pumpage, primarily for public supplies, has greatly expandedthe area in the southeast. This one area accounts for more than 25 squaremiles.

Other significant areas of dewatered rock, due primarily to pumpage forpublic supplies, are in the north central and south central areas of DuPageCounty. A few areas are associated with high bedrock surface elevations.

By 1979, water levels in DuPage County were below the bedrock surface overmore than 28 square miles. This is approximately 8.6 percent of the countyarea. The average thickness of dewatered rock is about 27 feet.

A 1966 water level-bedrock surface relationship map was also prepared(figure 11). Data from the 1966 and 1979 maps (figures 10 and 11) permitcalculation of the volume of rock dewatered during the 13-year period. Thisarea increased from approximately 19 to 28.5 square miles, or nearly 50percent. The average thickness of the additional 9.5 square miles of dewateredrock was 27.6 feet.

WATER LEVEL-BASAL SAND AND GRAVEL RELATIONSHIP .

Yields of dolomite wells have generally been higher in areas where theoverlying glacial drift contains moderate to thick deposits of sand and gravelin close proximity to the bedrock surface. Thus it is reasonable to expect aneven greater loss of well capacity where dewatering of sand and gravel hastaken place above dewatered bedrock.

In order to identify these areas, a review was made of well logs in theseareas of dewatered rock. Of particular interest were those logs that showed 10

16

Figure 9. Bedrock surface (after Zeizel et al., 1962)

17

Figure 10. Water level-bedrock surface relationship in 1979

18

Figure 11. Water level-bedrock surface relationship in 1966

19

or more feet of sand and gravel immediately above the bedrock. These data werethen compared with the 1966 and 1979 water level-bedrock surface relationshipmaps.

It is estimated that of the 28.5 square miles in DuPage County wheredolomite dewatering had occurred by 1979, approximately 10 square miles or 35percent of the area contained basal sand and gravel deposits at least 15 feetthick. The average thickness was approximately 30 feet. About 40 percent ofthis area, or 4 square miles, was dewatered during the period 1966-1979. Sincethe water level was below the bedrock surface over this entire area, it can beassumed the entire thickness of sand and gravel has been dewatered.

WATER BUDGET

The potential yield of an aquifer is defined as the maximum amount ofgroundwater that can be developed from a reasonable number of wells and wellfields without creating critical water levels or exceeding recharge. Aprevious study by Schicht et al. (1976) gave the potential yield of the shallowaquifers in DuPage County as 44.4 mgd. In order to verify previous analyses, agroundwater budget for DuPage County was prepared by balancing withdrawalsagainst potential yield plus the volume of material dewatered multiplied by thespecific yield.

Groundwater withdrawals from the shallow aquifers in DuPage Countyaveraged 36.7 mgd during the past 13 years; 34.3 mgd was from dolomite and 2.4mgd was from sand and gravel.

The volume of rock dewatered during the 13-year period is 7. x10 cu3

9

ft (9.5 square miles x 27.6 ft). At least an additional 3.3 x 10 cu ft (4.0square miles x 30 ft) of sand and gravel has been dewatered.

Todd (1959) defines specific yield as the ratio expressed as a percentageof the volume of water which can be drained by gravity after saturation to itsown volume. According to Todd, and based on the experience of the authors, aspecific yield of 20 percent for sand and gravel is reasonable. Lessinformation on the specific yield of dolomite is available. The equation givenbelow was solved to determine the specific yield of dolomite bedrock.

where

Q = average withdrawals, 1966-1979 = 36.7 mgd

R = potential yield = 39.4 mgd*

*Reduced by 5 mgd, which is estimated to be the recharge to surficialsand and gravels. The average withdrawals (Q) include sand and gravelpumpage which is from wells finished in basal sand and gravel.

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a = percent of potential yield diverted into cones ofdepression, 90 percent

vd = change in volume of dewatered rock = 7.3 x 10

9

cu ft

vs = change in volume of dewatered sand and gravel =3.3 x 10

9 cu ft

Ygd = specific yield for the dewatered rock = unknown

Ygs = specific yield = 0.2 for sand and gravel

7.48 = gal/cu ft

t = 13 years, 1966-1979 x 365 days per year

The specific yield of the dolomite bedrock was computed to be 0.017. Thiswould appear to be a reasonable value when compared with the value of 0.03given by Prickett et al. (1964) for the Chicago Heights area.

Pumpage over most of DuPage County is in excess of the potential yield.In order to balance the demand with the available supply, approximately 5.9billion gallons has been withdrawn from storage within the dolomite and sandand gravel aquifers. This is equivalent to an average pumpage of 1.2 mgdduring the past 13 years.

WATER QUALITY

During the summer of 1979, approximately 282 public, commercial, andprivate domestic water wells finished in the Silurian dolomite aquifer inthe study area at depths ranging from 66 to 500 feet (see figure 12) weresampled to obtain information on the areal distribution of the major dissolvedinorganic chemical constituents and total organic carbon. Roughly 44 percentof the samples collected were from public supplies, 39 percent were fromindustrial or commercial wells, and the remaining 17 percent were from privatedomestic wells. Data for total dissolved solids (TDS), hardness (as CaC03),sulfate, chloride, sodium, total organic carbon (TOC), fluoride, total iron,and nitrate were plotted and hand-contoured.

There are many factors which can affect the chemical composition of awater sample collected from a particular well, and it is impossible todelineate all of the conditions which may be significant for all wells sampled.However ) some general comments regarding groundwater quality sampling areappropriate.

Samples collected from high capacity wells, such as most public andindustrial water supply wells, represent an "average" groundwater quality for a

21

Figure 12. Frequency distribution of well depths

cone of influence which may be quite large. Leachate from a pollution sourcelocated in a well’s cone of influence may represent a very small portion of thewater produced by the well. Significant dilution of the polluted recharge bylarge volumes of Less mineralized water may result in a sample with lowconcentrations of chemical constituents. In these cases, the water quality inthe immediate vicinity of the pollution source is masked.

Similar to this areal averaging, vertical integration also may occur aswater from a more highly mineralized production zone is diluted by water fromother less mineralized water-yielding zones open to the well. When comparingdata from many wells, an undefined amount of error is introduced because thevolume of aquifer tapped and the effects of dilution vary among the wellssampled.

Although the results of the sample analyses are treated as point values,it is important to understand that in reality they represent averageconcentrations for a certain unknown volume of the aquifer which primarily

22

depends on the depth of the well in question, the hydraulic conductivity of theaquifer, and the rate of pumping. Despite these potential sources ofvariability and uncertainty, the results of the sample analyses can providevaluable information regarding present-day patterns in the regionaldistribution of the chemical parameters of interest.

The geohydrologic conditions which exist in northeastern Illinois resultin a moderate to high potential for contamination of the Silurian dolomiteaquifer. In fact, if the lowest concentrations measured in the study area areassumed to represent base-line conditions, the results of the chemical analysisof the collected samples reveal significant groundwater quality degradationin the southeastern quadrant of the study area.

Concentrations of total dissolved solids (TDS), hardness (as CaC03),sulfate, chloride, sodium, and total iron in samples from this region are high.In many cases they are several times greater than the applicable primary(mandatory) or secondary (suggested) standards for finished drinking water. Ona local scale, higher values for most of the nine chemical parameters ofinterest are also found in the immediate vicinity of the major municipalitiesin the central and western portions of the study area.

Of the nine parameters studied, only fluoride and nitrate exhibited arealdistribution markedly different from the other seven constitutents. Fluorideconcentrations were highest in the southwestern corner of the study area whilenitrate was greatest in the south and south-central regions. Generally, thelowest concentrations of most of the nine parameters studied are located in thewestern and southwestern regions between the Fox River and the West BranchDuPage River.

The areal distributions of the chemical parameters indicate thatgroundwater quality in the shallow dolomite aquifer is related to the densityof development of aquifer pumpage and of the overlying land surface. That is,the greatest concentrations generally are found in those areas with thegreatest development, for example in the southeastern portion. Conversely, thelowest concentrations seem to occur in those areas where the land surface andthe aquifer itself are less densely developed. In these less developed areas,there are no large zones of influence for wells to collect "pollutants" fromlarge areas at the land surface, and there are fewer sources of "polluted"recharge.

The age of development may also be a factor. For example, the highlymineralized southeastern portion has been heavily developed for severaldecades, while a newly developed area near Glendale Heights is just beginningto exhibit higher concentrations of minerals.

Notably, aquifer dewatering is occurring in several of those areasexhibiting the "worst" groundwater quality, though a causal relationship is notnecessarily indicated. The effects of aquifer dewatering on groundwaterchemistry are complex, but it has been shown that the aeration of a volume ofaquifer which was previously under anaerobic conditions can lead to increasesin certain dissolved minerals.

23

In addition, dewatering can result in markedly decreased well yields fromthe Silurian dolomite, as the most productive part of the aquifer is usually inthe upper zones of the formation. If the higher concentrations in thesoutheastern areas are due to one or more surface sources of pollution, thenthe decrease in well yields caused by the dewatering of the aquifer couldresult in less dilution of the highly mineralized recharge water.

Abandoned and active landfills may be sources of polluted recharge in thestudy area. In the past, common practice was to utilize existing excavationsfor waste disposal. Because their potential for polluting groundwater was notfully recognized, quarries and gravel pits often were considered "ideal" fordisposal of municipal and/or industrial waste materials. Disposal of wastesdirectly into or onto the Silurian dolomite aquifer near the study area hasbeen documented in several cases (Cunningham et al., 1977; Maynard, 1977;Shuster, 1976) and probably has occurred in numerous undocumented instances.These waste disposal sites, combined with those in the glacial drift, may beaffecting the chemical quality of the groundwater contained in thehighly-creviced dolomite aquifer.

It should be noted that those areas which are the most severely dewateredalso are areas where the glacial deposits are thin and the dolomite aquifer isvery close to the land surface. Less attenuation of non-conservative solutesoccurs in areas of thin drift. Contaminants from the surface may enter thedolomite aquifer more quickly and move rapidly toward points of withdrawal ornearby streams.

The higher concentrations of dissolved minerals in the southeasternportion of the study areas may be related to the thickness of the glacial driftcovering the dolomite aquifer, the dewatering of the aquifer, and thedevelopment of the overlying land surface. Conversely, a portion of thelow TDS area in the southwestern region also has been dewatered (see figure13). However, the dewatering has not affected the water quality, implying thatthe degree of land and aquifer development and the presence of pollutionsources may be more important than dewatering as determinants of groundwaterquality in the study area.

It is obvious that a combination of several factors are responsible forthe areal distribution of groundwater quality parameters. It must be kept inmind that the effects of human activities have been superimposed upon the"natural" patterns of chemical distribution that existed before development.The resulting composite patterns have been further disturbed by otheractivities such as heavy pumping and dewatering, quarrying, and mining of sandand gravel deposits. The extreme variability in the geohydrologic propertiesof fractured dolomite also adds to the difficulty in interpreting chemicaldata. In addition, the pre-development chemical conditions in the aquifer arevirtually unknown, and it is difficult to determine the significance of thenumerous factors affecting the water quality patterns of the aquifer.

Regardless of these difficulties, the collected data reveal water qualityproblems for a significant portion of the study area which eventually mayaffect water treatment costs and aquifer usability. In light of these results,

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Figure 13. Frequency distribution of total dissolved solids and hardnesscontents in shallow dolomite wells

a more detailed investigation of the groundwater quality deterioration in thesoutheastern portions of the study area may be warranted.

Discussions of individual water quality parameters are presented in thefollowing subsections.

Total Dissolved Solids (TDS)

The total dissolved solids (TDS) in samples collected for this studyranged from 259 milligrams per liter (mg/L) (Oswego) to 1832 mg/L (LaGrangePark), with a median value of 625 mg/L. Ten percent of the samples contained388 mg/L TDS or less and 10 percent had more than 968 mg/L (see figure 13).The USEPA has suggested that TDS concentrations in excess of 500 mg/L may haveundesirable effects on the aesthetic qualities of drinking water. Approxi-mately 70 percent of the samples exceeded this secondary drinking waterstandard.

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The highest TDS values are found in the southeastern section of the studyarea in a Y-shaped zone centered in the Hinsdale-LaGrange-Indian Head Park-BurrRidge region (see figure 14). The TDS values in this area generally aregreater than 1000 mg/L, with most of the groundwater containing more than 1200mg/L. This area of higher TDS values corresponds very closely to an area inwhich the dolomite has been dewatered (see figure 10).

High TDS concentrations also are found in the Elmhurst-Addison-Lombardregion. Most of this area exhibits TDS values greater than 1000 mg/L withlocal highs of more than 1200 mg/L. These highly mineralized areas are verynear to areas where the bedrock has been dewatered; however, the correspondenceis not quite as strong as in the Indian Head Park area described above.

Another area with high TDS concentrations is the Lisle-Downers Grove-Darien region. Total dissolved solids are greater than 800 mg/L with smallareas greater than 1000 mg/L. Much of this high TDS region also has beendewatered, as shown by figure 10.

Scattered smaller areas where high TDS concentrations are found includeArlington Heights (1192 mg/L), Aurora (912 mg/L), Elgin (909 mg/L), andNaperville (888 mg/L).

The lowest concentrations of TDS occur in the southwestern portion of thestudy area between the Fox River and the West Branch DuPage River. This areagenerally is characterized by low density development and rural land uses.Lower TDS values also are found in a similar area of low density developmentsouth of and including the Bartlett-Hanover Park region in the northwestportion of the study area.

Hardness (as CaCO3)

Hardness concentrations in the dolomite aquifer ranged from 134 mg/L(Oswego) to 1360 mg/L (LaGrange Park) with a median value of 481 mg/L, which isconsidered very hard. Ten percent of the samples had 299 mg/L hardness (asCaC03) or less and 10 percent had more than 690 mg/L (see figure 13). Thedistribution of hardness (see figure 15) is very similar to that for TDS, whichis to be expected since, in Illinois, hardness usually is the major contributorto TDS.

The area of highest hardness concentrations coincides with the area ofhighest TDS concentrations in the southeastern portion of the study area. TheY-shaped zone of highest concentrations is centered in the Hinsdale-LaGrange-Indian Head Park-Burr Ridge area and reaches south as far as theArgonne National Laboratory, All of this region is characterized by hardnessconcentrations greater than 600 mg/L, with most of the area exhibiting valuesof over 800 mg/L. Smaller zones of greater than 1000 mg/L are found in theWestern Springs area (1190 mg/L) and the LaGrange Park area (1360 mg/L).

High concentrations of hardness are also characteristic of theLombard-Elmhurst region in the east-central portion of the study area. In this

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Figure 14. Generalized areal distribution of total dissolved solids

in the shallow dolomite aquifer

27

Figure 15. Generalized areal distribution of hardness (as CaC03) in

the shallow dolomite aquifer

28

region, hardness concentrations greater than 600 mg/L are common. Smallerpeaks of over 800 mg/L are noted in Lombard (945 mg/L), Elmhurst (840 mg/L),and Bensenville (812 mg/L). Local highs of greater than 700 mg/L are locatedin the Elk Grove Village area and near Wheaton and Glen Ellyn.

Similar to the TDS distribution map (see figure 14), the lowest hardnessvalues generally occur in the western and southwestern parts of the study areabetween the Fox River and the West Branch DuPage River. In these regions,hardness values are typically less then 400 mg/L although small areas ofgreater than 600 mg/L occur locally near Aurora, West Chicago, Naperville, andStreamwood. The regions of lower hardness values generally occur in areas oflow density development of the aquifer and the overlying land surface.

Sulfate

Like TDS, hardness, and many other minerals, sulfate occurs naturally ingroundwater, mainly as a result of chemical reactions with the aquifermaterials. Sources of sulfate associated with human activities includeleachate from solid waste and effluent from sewage disposal.

Sulfate concentrations in groundwater samples from the dolomite aquiferranged from 0.1 mg/L (Wayne) to 864 mg/L (LaGrange Park) with a median value of166 mg/L. About 10 percent of the samples contained less than 49 mg/L, while10 percent contained more than 396 mg/L sulfate (see figure 16). The USEPA hassuggested that sulfate concentrations in excess of 250 mg/L may have anundesirable effect on the aesthetic qualities of drinking water. About 25percent of the samples exceeded this secondary standard.

Generally, sulfate concentrations increase from west to east across thestudy area with the highest concentrations located in the southeastern portionin a Y-shaped region stretching from LaGrange Park to Argonne NationalLaboratory (see figure 17). Concentrations in this area are commonly greaterthan 400 mg/L, with smaller areas of over 500 and 600 mg/L sulfate.

High sulfate concentrations are also found in the Schaumburg-Elk GroveVillage-Arlington Heights region located in the northeastern section of thestudy area. Values are greater than 400 mg/L over most of this area with asmaller area exhibiting concentrations greater than 500 mg/L.

Isolated high values occur at Bensenville (716 mg/L), Lombard (472 mg/L),Elmhurst (418 mg/L), Lisle (317 mg/L), Bolingbrook (310 mg/L), and theWheaton-Glen Ellyn area (342 to 365 mg/L).

The lowest sulfate values are found in the western portions of the studyarea, especially the southwest, between the Fox River and the West BranchDuPage River. In these areas, sulfate concentrations less than 100 mg/L arecommon and most of the areas have values less than 200 mg/L.

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Figure 16. Frequency distribution of sulfate content inshallow dolomite wells

Chloride

The concentrations of chloride in the samples from the dolomite aquiferranged from 0 to 450 mg/L (Elmhurst) with a median of 22 mg/L. Ten percent ofthe samples had 1 mg/L chloride or less and 10 percent had more than 128 mg/L(see figure 18). The USEPA has suggested that chloride in excess of 250 mg/Lhas an undesirable effect on the aesthetic qualities of drinking water. Lessthan 1 percent of the samples exceeded this secondary drinking water standard.The distribution of chloride values (see figure 19) is similar to that of TDSand many of the controlling factors are the same. However, road salt storageand application may be a major contributing factor in the elevated levels of

chloride Found in the study area, especially where sodium concentrations areunusually high. Septic tank effluent, landfill leachate, and water softenerbrine are also potential sources of chloride.

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Figure 17. Generalized areal distribution of sulfate in the

shallow dolomite aquifer

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Figure 18. Frequency distribution of chloride and sodium inshallow dolomite wells

As is the case in the distribution of TDS and hardness, the highestchloride concentrations generally are located in the southeastern quarter ofthe study area. The highest concentrations, typically greater than 150 mg/L,are found in the Hinsdale-Western Springs-LaGrange-Indian Head Park region.Another area of high chloride concentrations is located nearby in theLombard-Villa Park-Elmhurst region. These areas are traversed by several majorstate and interstate highways such as Interstates 294, 55, and 290. Because ofthe high density of highways in this area, deicing salt application and storagemay be a major cause of the high chloride concentrations in this portion of thestudy area.

Increasing chloride trends were discovered and reported previously forSilurian dolomite public water supply well fields at LaGrange and Naperville(Gibb and O'Hearn, 1980). The major cause of these trends is suspected to beroad salt storage and application. One co-author of this report counted 5uncovered salt stockpiles adjacent to I-294 in one h-mile segment near thestudy area during the spring months of 1981.

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Figure 19. Generalized areal distribution of chlorides in the


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A third area of high chloride is centered on the Glen Ellyn-Lisle-DownersGrove region located just west of the area mentioned above. This area ischaracterized by chloride in excess of 150 mg/L with concentrations greaterthan 100 mg/L extending south to the Bolingbrook area. The two high chlorideregions described above are separated by a small area near Westmont wherechloride values are less than 50 mg/L.

Isolated values of high chloride are found at Elgin (235 mg/L), NorthAurora (200 mg/L), Aurora (143 mg/L), and Naperville (105 mg/L).

The lowest chloride concentrations are located in the western portions ofthe study area, especially the southwestern region. In these areas, generallylying between the Fox River and the West Branch DuPage River, chlorideconcentrations are commonly less than 10 mg/L with many samples containingvirtually no chloride. There is a marked contrast between the lowest chlorideconcentrations in the western portion and the highest concentrations in theeastern portion of the study area.

Sodium

Potential sources of sodium in the groundwater environment include sewage,industrial waste, deicing compounds (road salt), and the aquifer materialitself. Where high sodium concentrations are accompanied by high chloridevalues, such as the southeastern corner and numerous other portions of thestudy area (see figure 19), improper road salt storage and application may be acontrolling factor. It should be noted that sodium, like chloride, appears tobe directly related to the density of highways in the study area, increasingboth from west to east and in the vicinity of large municipalities. Highsodium concentrations can also result from collecting samples after the waterhas been softened. The authors have attempted to screen the data for anysamples where such occurrences are suspected.

Sodium concentrations in the dolomite aquifer ranged from 4.1 mg/L(Naperville) t o 317 mg/L (Glendale Heights) with a median value of about30.2 mg/L. Ten percent of the samples contained less than 10.1 mg/L and 10percent contained more than 73.2 mg/L sodium (see figure 18). Presently, thereare no standards for sodium in drinking water; however, people on a sodium-restricted diet should be aware of the amount of sodium ingested daily viatheir water supply.

Sodium concentrations generally increase from northwest to southeastacross the study area with the highest concentrations centered in theHinsdale-Western Springs-Indian Head Park region (see figure 20). Sodiumconcentrations greater than 75 mg/L are common in this area but concentrationsgreater than 100 mg/L are found near Western Springs (246 mg/L) and Indian HeadPark (134 mg/L).

West of this area, another area of elevated sodium content area iscentered in the Glen Ellyn-Lisle region and stretches as far south asBolingbrook. Sodium concentrations in this region typically are greater than

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Figure 20. Generalized areal distribution of sodium in the


35

50 mg/L, with values greater than 75 mg/L found over much of the area. Smallpeaks within this area are located at Bolingbrook (186 mg/L), Glen Ellyn(171 mg/L), and Lisle (104 mg/L).

Between the two high-sodium areas described above there is a small area oflower concentrations centered near Westmont where the sodium content is lessthan 25 mg/L.

Elevated sodium concentrations are also found in the Aurora-Bataviaregion, where values over 50 mg/L are typical. Local highs of greater than100 mg/L are found near Aurora (142 mg/L) and North Aurora (108 mg/L). Greaterthan 40 mg/L sodium is found in virtually all areas east of Salt Creek and in asmall area in the Glendale Heights region. In addition, isolated high sodiumconcentrations are found at Elmhurst (202 mg/L) and Elgin (126 mg/L).

The lowest concentrations of sodium typically are found between the FoxRiver and the West Branch DuPage River, except in those areas of higherconcentration described above.

Total Organic Carbon

The collection and analysis of the water samples for total organic carbon(TOC) probably resulted in the loss of most or all of the volatile components.Therefore, the data presented are more representative of the nonvolatilefraction and, as such, may be as much as 25 to 50 percent lower than the actualTOC contained in the aquifer.

The total organic carbon content of the groundwater samples from theSilurian dolomite aquifer ranged from 0.2 (near Naperville) to 16.5 mg/L (nearOak Brook) with a median value of 1.6 mg/L. Ten percent of the samples contained0.9 mg/L or less and 10 percent contained more than 2.7 mg/L (see figure 21).There are no TOC standards for drinking water; however, concentrations inIllinois groundwater typically are less than 3.0 mg/L with most values less thanor equal to 2.0 mg/L.

Yost of the study area exhibits TOC concentrations less than 3.0 mg/L (seefigure 22). Higher concentrations are found in the South Elgin-Wayne-WestChicago area where concentrations greater than 4.0 mg/L are common and values ashigh as 6.1 mg/L (near Wayne) are recorded.

Total organic carbon values 5.0 mg/L and greater are also found in thevicinity of Villa Park and Downers Grove. Concentrations of 4.2 and 16.5 mg/Loccur in the vicinity of Oak Brook, although surrounding values are typically3.0 mg/L or less. A local peak of 4.6 mg/L is found in the vicinity of Wheatonlocated in the center of the study area.

The lowest TOC values are located in the extreme southern portion, in asmall area immediately south of an isolated high at Elgin (12.5 mg/L), and insmall isolated pockets across the study area. Concentrations of TOC in theseregions are less than 1.0 mg/L.

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Figure 21. Frequency distribution of total organic carbon inshallow dolomite wells

The highest TOC values generally occur as isolated phenomena surrounded bylower values. This characteristic of TOC distribution suggests that localsources of organic contaminants are responsible for these occurrences.

Fluoride

The concentration of fluoride in the dolomite aquifer in the study arearanges from 0.1 mg/L to 1.6 mg/L (Glen Ellyn), with a median value of 0.3 mg/L.Ten percent of the samples collected have 0.2 mg/L or less and 10 percent havemore than 0.7 mg/L (see figure 23). The maximum concentration allowed infinished public water supplies is 2.0 mg/L. Fluoride is occasionally added topublic water supplies as a tooth decay preventative when natural concentrationsare below 0.5 mg/L. In these cases, finished water is required to containbetween 0.9 and 1.2 mg/L fluoride. Other possible sources of fluoride ingroundwater are municipal and industrial wastes and fluoride-containing aquifermaterials.

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Figure 22. Generalized areal distribution of total organic carbon

in the shallow dolomite aquifer

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Figure 23. Frequency distribution of fluoride, total iron, and nitratein shallow dolomite wells

Contrary to the distribution of most other dissolved inorganics, the highestconcentrations of fluoride are located in the extreme southwestern corner of thestudy area, where values greater than 0.75 mg/L are common (see figure 24). Inthe same region, several values greater than or equal to 1.0 mg/L are reported,with the highest concentration in this area, 1.4 mg/L, found near Oswego.Isolated higher values also occur in the Glen Ellyn area (1.6 mg/L) and the NorthAurora area (1.0 mg/L). Sample contamination is suspected in the case of asample from Bolingbrook containing 28.0 mg/L fluoride. A sample with 5.5 mg/Lfluoride from a well near Glendale Heights is probably not representative ofwater from the dolomite aquifer since an extremely low hardness value in thissample indicates that the sample may have been collected after ion-exchangesoftening.

The lowest concentrations of fluoride generally are found in thesoutheastern quarter and the extreme northwestern corner of the studyarea.

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Figure 24. Generalized areal distribution of fluoride in the


40

Total Iron

The concentration of total iron in samples collected from the dolomiteaquifer ranges from 0.0 mg/L to 53.6 mg/L (near Downers Grove), with a medianvalue of 1.2 mg/L. Ten percent of the samples contained 0.1 mg/L total iron orless and 10% had total iron concentrations in excess of 4.3 mg/L (seefigure 23). Roughly 5 percent of the samples contained greater than 10.0 mg/Ltotal iron.

The maximum allowable concentration of total iron in treated samples frompublic water supplies is 1.0 mg/L. Concentrations in excess of 0.3 mg/L mayhave an undesirable effect on the aesthetic qualities of drinking water. Theconcentration of iron is affected by numerous factors including but not limitedto: geology and hydrology of the aquifer, presence of oxygen and other mineralcomponents, pH, redox potential (Eh), presence of organic materials, microbialactivity, and the turbidity of the sample. The variability of these conditionsfrom point to point within an aquifer results in large variations in the ironcontent of groundwater. In addition, landfill leachates from municipal andindustrial wastes can add significant amounts of iron to the groundwateralthough these contributions usually are difficult to separate from those ofnatural sources.

The highest concentrations of total iron generally are located in thesoutheast quarter of the study area in the Downers Grove-Oak Brook-Lombard-Glen Ellyn region (see figure 25). Total iron values in this regiontypically are greater than 5.0 mg/L. Of the 14 samples with greater than10.0 mg/L total iron, 7 of them are located in this region. Five of these 7samples contained iron in excess of 20.0 mg/L, and 2 samples exceeded 40.0 mg/L(Downers Grove-53.6 mg/L, Oak Brook-42.0 mg/L).

High total iron values are also found in the vicinity of Naperville(19.2 mg/L) just west of the Downers Grove-Oak Brook-Lombard region. A smallarea in the Batavia-North Aurora region exhibited iron values in excess of15 mg/L with a well near Batavia showing 24.3 mg/L. Isolated highconcentrations of total iron are located in the vicinity of South Elgin(13.0 mg/L), Winfield (13.0 mg/L), Mt. Prospect (12.6 mg/L), and Roselle(11.1 mg/L).

The lowest iron concentrations seem to be scattered throughout the studyarea, but generally are concentrated in the southern portions and along the FoxRiver, except near the major municipalities of Aurora and Elgin. Ironconcentrations are consistently lower between the Fox River and the West BranchDuPage River and in the northeastern corner of the study area where valuestypically are less than 3.0 mg/L.

Nitrate

The concentration of nitrate in samples collected from the Siluriandolomite aquifer ranges from 0.0 mg/L to 63.2 mg/L (south of Aurora) with a

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Figure 25. Generalised areal distribution of total iron in the


42

median value of 0.2 mg/L. More than 10 percent of the samples had nodetectable nitrate content and 10 percent contained at least 2.8 mg/L (seefigure 23). The maximum allowable concentration of nitrate in finished publicwater supply samples is 45.0 mg/L. Only 2 percent of the 282 raw water samplesexceeded this primary standard for finished drinking water.

Nitrates are formed naturally by bacteria during the aerobic decompositionof organic nitrogenous wastes such as those present in most domestic,municipal, and industrial wastewaters. Nitrates may also be leached fromsoils, especially where nitrogen fertilizer has been applied. High levels ofnitrate in a particular well may be an indication that the well itself isinadequately sealed from contamination by surface runoff. If numerous wells inthe same area contain higher concentrations of nitrate, a regional source ofcontamination may be present.

The highest concentrations of nitrate are found in the south-centralportion of the study area, particularly in the Romeoville-Bolingbrook-Plainfield region (see figure 26). In this area of moderate agriculturalactivity, nitrate concentrations in excess of 10.0 mg/L are very common, and 3samples contained more than 20.0 mg/L nitrate. A sample collected from a wellnortheast of Plainfield contained 62.7 mg/L nitrate. Directly north of thisregion, high concentrations of nitrate are found in the Downers Grove-Naperville region with typical values of over 10.0 mg/L. Isolated high valuesoccur south of Aurora (63.2 mg/L), near Elgin (31.0 mg/L) and South Elgin(17.1 mg/L), and in West Chicago (17.1 mg/L).

The rest of the study area generally has low values of nitrate with manysamples showing little or no nitrate as indicated by the concentrationfrequency distribution in Figure 23.

SUMMARY AND CONCLUSIONS

Groundwater withdrawals from the shallow dolomite aquifer in the westsuburban area of the Chicago metropolitan region have had a severe impact onwater levels and on the long-range capability of wells and well fields tocontinue to meet demands for increasing volumes of water by municipalities,industries, and individuals.

Shallow aquifer pumpage in the study area increased 92 percent during theperiod 1966-1978 and was 61.7 mgd in 1978; 57.3 mgd was from the dolomite, and4.4 mgd was from sand and gravel. Pumpage in DuPage County increased 91percent during the same period. The 1978 pumpage of 45.8 mgd was 103 percentof the potential yield.

As a result of the continual increase in pumpage, water levels havelowered considerably, and major sections of the aquifer system have been atleast partially dewatered. Well yields in several municipalities have declinedas the result of heavy pumpage. Large volumes of rock, sand, and gravel havebeen dewatered as water is withdrawn from storage within the aquifer to make upthe deficit between water demand and the potential yield.

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Figure 26. Centralized areal distribution of nitrate in the


44

In DuPage County, water levels in the shallow dolomite declined more than10 feet over an area of 98.4 square miles during the period 1966-1979. Thisincludes nearly 30 percent of the entire county. The average decline for the98.4 square miles was approximately 1.5 feet per year. Water levels declinedmore than 30 feet over 8.5 square miles.

The decline in water levels has resulted in water levels being loweredbelow the bedrock surface over approximately 9.5 square miles of DuPage Countysince 1966. An average of 27.6 feet of rock has been dewatered over this area.In addition to the dewatered rock, approximately 4 square miles of sand andgravel above the dewatered bedrock has been dewatered an average of about 30feet.

A volume of 7.3 billion cubic feet of dolomite and 3.3 billion cubic feetof sand and gravel has been dewatered in DuPage County during the period 1966-1979. Approximately 5.9 billion gallons of water has been withdrawn from theshallow aquifer system in excess of that available from natural recharge. Thisis equivalent to average pumpage of 1.2 mgd.

Schicht et al. (1976) predicted that the groundwater demand in DuPageCounty would be 97.2 mgd by the year 2000. If pumpage was in the same ratio asin 1978, shallow aquifer pumpage would be approximately 63 mgd. This would beapproximately 18 mgd more than the potential yield of these aquifers.Withdrawal of 18 mgd from storage within the aquifers would result in extremelysevere dewatering over extensive areas. Major decreases in yields would beexperienced in nearly all wells and well fields throughout the county.

The areal distribution of the chemical parameters studied indicates thatgroundwater quality in the shallow dolomite aquifer is related to the densityof development of the land surface and the aquifer. Areas of highest chemicalconcentration generally are found in areas with the heaviest development.Degradation of shallow dolomite water quality is particularly noticeable in thesoutheastern part of DuPage County. Concentrations of total dissolved solids,hardness, sulfate, chloride, sodium, and total iron in samples from this regionare high.

In addition to the southeastern portion of the study area, locally highvalues of all nine chemical parameters studied were found in the immediatevicinity of the major municipalities in the other portions of the study area.Generally, the lower concentrations of most of the nine parameters studied werefound in the western and northwestern regions between the Fox River and theWest Branch DuPage River.

Available data suggest that the age of land surface and aquiferdevelopment also is a factor in determining water quality for this aquifer.Relatively good quality water is found in the undeveloped and newly developedportions of the study area. The results of this portion of the study suggestthat deteriorating water quality will eventually affect water treatment costand aquifer usability. In light of these results, periodic studies of thistype are recommended to assess the effectiveness of water quality controlprograms and provide updated information on the chemical state of groundwaterfrom this important aquifer.

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REFERENCES

Cunningham J. A., R. G. Lukas, and T. C. Anderson, 1977: Impoundment of FlyAsh and Slag--A Case Study, Geotechnical Practice for Disposal of SolidWaste Materials, American Society of Civil Engineers, New York, NY,pp. 227-245.

Gibb, J. P., and M. O'Hearn, 1980. Illinois Groundwater Quality Data Summary,Illinois State Water Survey Contract Report 230, Urbana, Illinois.

Maynard, T. R., 1977: Incinerator Residue Disposal in Chicago, GeotechnicalPractice for Disposal of Solid Waste Materials, American Society ofCivil Engineers, New York, NY, pp. 773-792.

Prickett, T. A., L. R. Hoover, W. H. Baker, and R. T. Sasman, 1964: Ground-water Development in Several Areas of Northeastern Illinois. IllinoisState Water Survey Report of Investigation 47.

Sasman, R. T., 1974: The Future of Ground-water Resources in DuPage County.Ground Water, v. 12(5).

Schicht, R. J., J. R. Adams, and J. B. Stall, 1976: Water ResourcesAvailability, Quality, and Cost in Northeastern Illinois. Illinois StateWater Survey Report of Investigation 83.

Shuster, K. A., 1976: Leachate Damage Assessment: Case Study of the FoxValley Solid Waste Disposal Site in Aurora, Illinois, USEPA Report No.SW-514.

Suter, M., R. E. Bergstrom, H. F. Smith, G. H. Emrich, W. C. Walton, and T. E.Larson, 1959: Preliminary Report on Ground-water Resources of theChicago Region, Illinois. Illinois State Water Survey and GeologicalSurvey Cooperative Report 1.

Todd, D. K., 1959: Ground Water Hydrology. John Wiley & Sons, New York.

Zeizel, A. J., W. C. Walton, R. T. Sasman, and T. A. Prickett, 1962: Ground-water Resources of DuPage County, Illinois. Illinois State Water Surveyand Geological Survey Cooperative Report 2.

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verification of the potential yield and chemical quality

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